#include <bits/stdc++.h>
using namespace std;

#define int int64_t
#define endl "\n"

using PII = pair<int, int>;
using TII = tuple<int, int, int>;
template<class T>
using V = vector<T>;
using i64 = long long;
using i128 = __int128;
using u64 = unsigned long long;

#define FOR(i, a, b) for (int i = (int)(a); i <= (int)(b); i++)
#define FOR2(i, a, b, c) for (int i = (int)(a); i <= (int)(b); i += c)
#define FORD(i, b, a) for (int i = (int)(a); i >= (int)(b); i--)
#define FORD2(i, b, a, c) for (int i = (int)(a); i >= (int)(b); i -= c)
#define ALL(a) a.begin(), a.end()
#define RALL(a) a.rbegin(), a.rend()
#define lowbit(x) ((x)&(-x))
#define RE return;
#define Yes cout << "Yes" << endl;
#define YES cout << "YES" << endl;
#define No cout << "No" << endl;
#define NO cout << "NO" << endl;
#define pb push_back
#define eb emplace_back
#define fi first
#define se second
#define sz size()

void solve () {
    int n;
    string s;
    cin >> n >> s;

    string ans = "zzzzzzzz";

    FOR (i, 'a', 'z'){
        string p;
        p += char (i);
        if (s.find (p) == -1){
            cout << p << endl;
            RE;
        }
    }

    FOR (a, 'a', 'z'){
        FOR (b, 'a', 'z'){
            string p;
            p += char (a);
            p += char (b);
            if (s.find (p) == -1){
                cout << p << endl;
                RE;
            }

        }
    }

    FOR (a, 'a', 'z'){
        FOR (b, 'a', 'z'){
            FOR (c, 'a', 'z'){
                string p;
                p += char (a);
                p += char (b);
                p += char (c);

                if (s.find (p) == -1){
                    cout << p << endl;
                    RE;
                }
            }
        }
    }

    FOR (a, 'a', 'z'){
        FOR (b, 'a', 'z'){
            FOR (c, 'a', 'z'){
                FOR (d, 'a', 'z'){
                    string p;
                    p += char (a);
                    p += char (b);
                    p += char (c);
                    p += char (d);
                    if (s.find (p) == -1){
                        cout << p << endl;
                        RE;
                    }
                }
            }
        }
    }

}

signed main () {
    int Task = 1;

    ios::sync_with_stdio (false);
    cin.tie (nullptr);
    cin >> Task;

    while (Task--) {
        solve ();
    }

    return 0;
}